A crusher of the above-mentioned type may be utilized in order to crush hard material, such as pieces of rock material. It is desirable to be able to crush a large quantity of material in the crusher without risking that the crusher is exposed to such mechanical loads that the frequency of breakdowns increases.
WO 87/05828 discloses a method to decrease the risk of increased mechanical load and breakdowns resulting therefrom. The number of pressure surges above a certain predetermined level that arise in the hydraulic fluid that controls the position of the crushing head are counted. If the count of pressure surges exceeds a predetermined amount, the relative position of the crushing shells is changed so that the width of the crushing gap increases. Preferably, the number of times that the gap is increased during a predetermined time is also counted after which alarm is given if said number of times exceeds a predetermined amount.
The method disclosed in WO 87/05828 may to a certain extent reducing the risk of the crusher breaking down prematurely, but does not increase the efficiency of the crusher as regards the amount of crushed material per unit of time.